Lesson Overview 17.4 Molecular Evolution

THINK ABOUT IT An organism’s genome is its complete set of genetic information. The analysis of genomes enables us to study evolution at the molecular level.   When comparing DNA sequences of different organisms, the sequences may indicate how two species are related to one another, even if their body structures don’t offer enough clues.

Molecular Clocks When researchers use molecular clocks, they compare stretches of DNA to mark evolutionary time.   A molecular clock uses mutation rates in DNA to estimate the time that two species have been evolving independently.

Neutral Mutations as “Ticks” Think about a pendulum clock. They mark time with a swinging pendulum.   A molecular clock relies on a repeating process to mark time – mutations. Simple mutations occur all the time, causing slight changes in the sequence of DNA. Some mutations have major positive or negative effects on an organism’s phenotype. These mutations are under powerful pressure from natural selection. Neutral mutations have no effect on phenotype and tend to accumulate in the DNA of different species at about the same rate.

Neutral Mutations as “Ticks” Comparison of DNA sequences between species can show how many mutations occurred independently in each group. The more differences there are between the DNA sequences of the two species, the more time has elapsed since the two species shared a common ancestor.

Calibrating the Clock Using molecular clocks isn’t simple because there isn’t just one molecular clock in a genome. Because some genes accumulate mutations faster than others, there are many different molecular clocks that “tick” at different rates. These different clocks allow researchers to time different evolutionary events. Think of a conventional clock. If you want to time a brief event, you use the second hand. To time a long event, you use the minute or hour hand.   Researchers check the accuracy of molecular clocks by trying to estimate how often mutations occur. They compare the number of mutations in a particular gene in species whose age has been determined by other methods.

Gene Duplication Where did the 25,000 genes in the human genome come from? Scientists think they probably descended from a much smaller number of genes in the earliest life forms. How is this possible?   One way in which new genes evolve is through the duplication, and then modification, of existing genes.

Copying Genes Most organisms carry several copies of a gene or carry two copies of the same gene. Where do these extra copies come from? Homologous chromosomes exchange DNA during meiosis in a process called crossing-over.   Sometimes crossing-over involves an unequal swapping of DNA so that one chromosome in the pair gets extra DNA. That extra DNA can carry part of a gene, a full gene, or a longer length of chromosome. Sometimes, an entire genome can be duplicated.

Duplicate Genes Evolve Sometimes copies of a gene undergo mutations that change their function. The original gene is still around, so the new genes can evolve without affecting the original gene function or product.   A gene is first duplicated, and then one of the two resulting genes undergoes mutation.

Gene Families Multiple copies of a duplicated gene can turn into a group of related genes called a gene family.   Members of a gene family typically produce similar, yet slightly different, proteins. For example, your body produces a number of different molecules that carry oxygen. Several of these compounds – called globins – are hemoglobins. The globin gene family that produces them evolved, after gene duplication, from a single ancestral globin gene.

Developmental Genes and Body Plans There is a new research area nicknamed “evo-devo” because it studies the relationship between evolution and embryological development. Darwin himself thought that changes in the growth of embryos (like the mouse embryo below) could transform adult body shape and size. Researchers now study how small changes in Hox gene activity could produce the kinds of evolutionary changes we see in the fossil record.

Hox Genes and Evolution Hox genes determine which part of an embryo develops arms, legs, or wings. Groups of Hox genes also control the size and shape of those structures.   Small changes in Hox gene activity during embryological development can produce large changes in adult animals.

Change in a Hox Gene Insects and crustaceans are descended from a common ancestor that had many pairs of legs.   Crustaceans (such as brine shrimp) still have lots of legs. Insects, however, have only three pairs of legs.

Change in a Hox Gene Recent studies have shown that in insects, a mutation in a single Hox gene, called Ubx, “turns off” the growth of some pairs of legs.   Because of mutations in a single Hox gene millions of years ago, modern insects have fewer legs than modern crustaceans. A variant of the same Hox gene directs the development of the legs of both animals.

Timing Is Everything Each part of an embryo starts to grow at a certain time, grows for a specific time, and stops growing at a specific time. Small changes in starting and stopping times can make a big difference in organisms. For example, small timing changes can make a difference between long, slender fingers and short, stubby toes.